Two hundred and forty adult and sub-adult Paracentrotus lividus were divided into two major groups (adults, sub-adults) based on their diameter differences (above or below 40 mm) and allocated to baskets suspended from the walkways of finfish cages, in South Evoikos Gulf, Central Greece. Each group were fed a commercial finfish diet, while their respective controls remained unfed during the whole duration of the experiment. Total weight and gonad weight of all treatment groups increased while the respective weights of the control groups decreased by the end of the experiment. In addition, gonad indices of the treatment groups increased by the end of the experiment, except for the sub-adult groups. No mortalities were observed in the treatment groups, while the low mortalities in the control groups (1–1.5%) can be attributed to the nutrient status of the surrounding waters and the increased plankton populations and sedentary organisms. It has therefore been shown that fish/sea urchin polyculture in Greece can be low risk and beneficial for the producer system, as long as prevailing water quality and environmental aquaculture standards are kept.